1. Field of the Invention
The present invention relates to an optical transmission line enabling Raman amplification of an optical signal when pumping light is supplied thereto, a method of making this optical transmission line, and an optical transmission system using this optical transmission line.
2. Related Background Art
An optical fiber amplifier amplifies optical signals so as to compensate for their loss when they propagate through an optical transmission line in an optical communication system. This optical fiber amplifier comprises a light-amplifying optical fiber and pumping light supply means. Namely, when pumping light of a predetermined wavelength is supplied from the pumping light supply means to the light-amplifying optical fiber, and an optical signal is inputted to the light-amplifying optical fiber, thus inputted optical signal is amplified by the light-amplifying optical fiber, and the amplified signal is outputted therefrom.
Known as such optical fiber amplifiers are one in which an optical fiber whose optical waveguide region is doped with a rare-earth element (e.g., Er element) is used as the light-amplifying optical fiber (hereinafter referred to as “rare-earth element doped optical fiber amplifier”) and one in which Raman amplification is utilized (hereinafter referred to as “Raman amplifier”). While the rare-earth element doped optical fiber amplifier is disposed in a repeater or the like as being formed into a module, which is only used as a discrete amplifier, the Raman amplifier can not only be used as a repeater but also amplify optical signals in an optical transmission line (optical fiber) through which the optical signals propagate, which is so-called distributed amplifier. Therefore, if Raman amplification is utilized, then not only the effective loss in the optical transmission line can be reduced, but also optical Kerr effects can be restrained from occurring due to the fact that the power of optical signals at each location in the optical transmission line becomes too high.
For example, Japanese Patent Publication No. 2617612 discloses a technique using an Er element doped optical fiber together with Raman amplification, so as to attain a uniform power distribution of optical signals in the longitudinal direction of the optical fiber. On the other hand, reference 1—L. F. Mollenauer, et al., IEEE J. of Quantum Electron., Vol. QE-22, No. 1, pp. 157–173 (1986)—describes Raman amplification caused by bidirectional pumping, thus disclosing a technique for lowering the effective loss in optical transmission lines. Also, reference 2—H. Masuda, et al., EOC'99, II-146 (1999), reference 3—H. Suzuki, et al., ECOC'99, PD2-4 (1999), and reference 4—T. N. Nielsen, et al., ECOC'99, PD2-2 (1999) disclose techniques for Raman amplification of optical signals by use of a dispersion-shifted optical fiber or non-zero dispersion shifted fiber as an optical transmission line.
While U.S. Pat. No. 5,778,128 discloses the hybrid transmission line that is composed of dispersion compensated fiber inserted between two single mode optical fibers with equal length.